1. Field of the Invention
The present invention relates to an ink set for an image recording apparatus, which comprises a plurality of inks which are used to record images with respect to a recording medium.
2. Description of the Related Art
Various kinds of image recording apparatuses are known. In recent years, an ink jet printer adopting an ink jet recording mode has been extensively used since it is relatively inexpensive and small in size. Such an ink jet printer discharges ink droplets toward a recording medium, thereby recording an image on the recording medium.
For an ink jet printer, there have been adopted various recording methods in order to change in lightness of colors to be recorded. As one recording method, the number of dots per unit area of ink dots spotted on a recording medium is increased or decreased. When a gray scale is recorded based on this recording method, a part with a low gradation level is recorded with a low printing percentage, and a part with a high gradation level is recorded with a high printing percentage. Therefore, when recording of a part with a low gradation level is attempted by using an ink with a high density, a recorded image has a grainy appearance.
It is to be noted that the term “printing percentage” means the number of ink dots per unit area recorded on a recording medium. For example, when an image is recorded with a resolution of 360 dpi, recording all ink dots in 360-by-360 lattice points arranged in one square inch means a printing percentage 100%.
As another conventional recording method for disappearing the grainy appearance of the ink dots into the shade, there is a recording method using inks with different densities. In this method, there is used an ink set for an image recording apparatus, which has a plurality of types of inks. This ink set for an image recording apparatus comprises a normal ink with a high density and a light ink with a density lower than that of the normal ink, in accordance with each color.
Incidentally, in this specification, when using a plurality of inks at the time of recording one color, a given ink is determined as a first ink, and an ink with a density different from that of the first ink is determined as a second ink. Therefore, in the ink set for an image recording apparatus, the normal ink is determined as the first ink, and the light ink is determined as the second ink.
When recording a 256-gradation gray scale by using these inks, an ink jet printer records a part with a high gradation level by mainly using the normal ink, records a part with a low gradation level by mainly using the light ink, and records a part with an intermediate gradation level by using both the normal ink and the light ink. As a result, the ink jet printer represents the change in lightness of the gray scale. More particularly, when recording a gray scale from a low gradation, only the light ink is used at the beginning as shown in FIG. 4. At this moment, the printing percentage is gradually increased as the gradation level is increased. When the printing percentage of the light ink becomes 100%, the printing percentage of the light ink starts to be decreased, and the printing percentage of the normal ink starts to be increased in turn. It is to be noted that, in the recording, the gradation level becomes maximum when the printing percentage of the normal ink becomes 100%. That is, the gradation level when the printing percentage of the normal ink becomes 100% is 255.
As described above, in the method, a timing to start printing using the normal ink (first ink) is when the printing percentage of the light ink (second ink) starts to be decreased after reaching a predetermined quantity. Based on the above, the start timing is when the printing percentage of the light ink starts to be decreased after the printing percentage reaches 100%. That is, printing using the normal ink starts immediately after the printing percentage of the light ink reaches 100%.
It is to be noted that, in the case of recording a gray scale, the gradation level when the first ink starts to be used is referred to as the “printing start level” in this specification. In other words, the term “printing start level” means a timing (gradation level) with which printing using the first ink starts when the printing percentage of the second ink starts to be decreased in the case of recording a gray scale by using two inks with different densities. Therefore, when there is a description “the printing start level is set to the gradation level (128)”, this means that the gradation level at the time of the start timing is 128. It is to be noted that this example means printing using the normal ink starts at the gradation level (128).
In the gray scale recorded by using the ink set for an image recording apparatus, since a part with a low gradation level is recorded by using the light ink as described above, the ink shade appears grainy.
In the ink set for an image recording apparatus using the above-described method, the light ink is generally produced by diluting the normal ink having a high density by using a solvent. In the thus manufactured light ink, its hue is changed because of the solvent. As a result, at the printing start level, the hue of the normal ink recorded on the recording medium largely differs from the hue of the light ink.
In more detail, when recording the gray scale in such a manner that the gradation level is gradually increased, printing using the light ink as well as printing using the normal ink starts at the printing start level. That is, at the gradation level higher than this printing start level, the gray scale is formed of ink dots of two inks having different hues.
Therefore, in the gray scale, color skipping becomes distinctive with the printing start level at a boundary. The term “color skipping” means that an observer feels a difference in lightness as well as a difference in hue when comparing close gradation areas with different color lightness even though the gray scale is represented by using one color.
Color skipping becomes distinctive with the printing start level at the boundary because the hue suddenly varies with the printing start level at the boundary. That is, when the conventional ink set for an image recording apparatus is used, the gray scale is constituted by using both the first and second inks having different hues. As a result, the hues of the inks greatly differ with the printing start level at the boundary, and hence color skipping occurs.
The ink set for an image recording apparatus by which the color skipping becomes distinctive in the gray scale is hard to record a natural image with the high quality.